Satin white and method of making the same



/ Pat ented Feb. 9,1943

SATINVWHITE AND METHOD OFMAKING THE SAME Gerald Haywood, Western Port, Md., assignor to -West Virginia Pulp and Paper a corporation of Delaware York, N. Y.,

Company, New

No Drawing. Application June 12, 1942.

Serial No. 446,825

This invention relates to, improvements in a product of the type commonly designated satin white and also to improvements in the method of producing the same. More particularly, the improved product is an intimate, coprecipitated mixture of satin white and calcium sulfit'e, the satin white largely predominating. While the precise, chemical nature of the product has notbeen definitely determined it is possiblethat it is a chemical combination of satin white and calcium suliite.

This application is a continuation-in-part of my pending application, Ser. No, 351,113, filed August 3, 1940..

The precise character of satin white has been subject to some discussion among chemists. It is now generally accepted, however, that this product, which is usually formed by the reaction of a concentrated solution of iron-free alum with milk of lime, is calcium sulfo-aluminate and may be represented by the formula:

scaonhoiacaso simo.

In the ordinary procedure for the production of satin white, care must be exercised to eliminate the usual iron/content from the alum employed, otherwise the resulting product will be discolored.

I The ferric hydroxide formed in the reaction will impart a yellowish or other objectionable color to the material. Accordingly, the production of a pure white product, suitable for use as a pigment, for example in the coating composition applied to coated papers, is rather expensiveand, therefore, it is usable only in the .coating of relatively high-grade papers. I It has been a primary object of the present invention to simplify the procedure employed in the production of satin white and to lessen the expense involved so that this product may be used more extensively in the coating of papers, including those of cheaper grades. Toward this end the improved method makes it possible to employ ordinary alum from which the usual iron content has not been remove 1 Another object of the invention has been to produce an improved product which is primarily satin white, or calcium sulfo-aluminate, but which contains a smaH'amount not exceeding about of calcium sulfite. It has been found that the presence of the calcium sulfite in no way detracts from the'brightness, gloss and opacity-producing power of the material but rather enhances these properties and at the same time presents other distinct advantages. For example, the mixture or combination of alum.

satin white and calcium sulfite has been found to flow muchinore freely than satin'white alone. This facilitates the application of a coating embodying satin white to the surface of paper to be coated. Moreover, it has been foundthat a mixture of, say 1 to 10% calcium sulfite and 90 to 99% satin white, on anair -dry basis, may be used in the coating of paper with a smaller amount of casein or other adhesive substance than is required by ordinary satin white alone. Pure satin white requires a relatively large amount of casein to bind it firmly to the fibers 'of the sheet, but for some reason, the presence of a comparatively small amount of calcium sulfite greatly reduces the casein requirement.

In the production of the improved satin white, which, as indicated above, is in reality a mixture or combination-of satin white-and calcium sulfite, the two constituents are precipitated simultaneously. This not only brings about a desir-' able, intimate intermixture of the two ingre dients in the desired proportion but it accomplishes this result at aminimum of expense and enables the use of a relatively low grade otalum which has not been treated to remove its iron content. Moreover, there may be some chemical combination between the constituents due to coprecipitation, since the mixture of the two ingredients separately formed does not have the same improved characteristics.

- In the conduct of the improved process a solution of unpurified alum is subjected to the action of gaseous $02 until it becomes saturated with $02. The alum solution used for this purpose could be a saturated solution, in which case it would contain approximately one pound of alum for each pound of water. This would be particularly satisfactory if such a solution would also dissolve the full amount (rt/SO: normally soluble in the water present, 1. e., about 10% by weight. However, it has been found that when the water is saturated with alum, it will dissolve much less S02 than is soluble in plain water; Ordinanly, therefore, it is desirable to introduce more S02 than will dissolve "in a saturated solution of alum, in order to-increase the calcium sulfitecontent of the resulting mixture. For this'purpose it is preferable to employ an alum solution containing between 30 ands 45%, byrweight, of

In a typical operation, in accordance with the invention, an alum solution consisting of 45% alum by weight and Water was saturated with sulfur dioxide. It was ,found that a liter of the alum solution weighing 1310 grams would However, the presence of the alum, with its common ioneffect, served to reduce the solubility of sulfur dioxide. This solution of alum and S: was then added to milk of limecontaining sufficient lime to provide a ratio between CaO and S0: of 1.67. This quantity of lime is in excess, by about 10%, of that required to react' fully with the alum white. of the formula hereinabove given, and

, calcium sulflte. The resulting mixture, therecontained a small amount of the satin white produced was fore, must have free lime, or else and S0: to produce satin" not of the composition indicated by the formula.

analyzed and found air-dry A sample of the mixture was to contain 3.56% calcium sulflte on the basis. composition, was found to possess greater brightness'and gloss-producing powers than straight satin white produced from the same lime and alum without the addition of S02. It was also found to, have certain other advantages from the standpoint of such use. .Its viscosity in a water suspension containing a predetermined amount of water in relation to the weight of the solids was lower, i. e., its fluidity was greater, than for a corresponding suspension of .the straight satin white. Moreover, its casein requirement was less than for the straight satin white. Thus, in producinga coating composition sufficiently sizedto be good to a No. 5A graded Dennison wax, a substantially smaller amount of casein was required for the improved mixture than forthe straight satin white, For example,a typical sample of the latter required of casein, based upon the air-dry weight of the satin white, to make it safe to a No. 5A wax while the improved mixture required only 27% of casein, ;or the same purpose, the coatings being of equal weight, about 18 lbs. per ream. The increased fluidity of the improved mixture .not only simplifies the intermixture of the lime and alum and reduces the amount; of

water required to be used with it in the produc tion of a coating composition, and hence reduces the amount of water which must be eliminated in the-'driers, in the process of forming coated papers, but it alsoappears to hasten the reaction between the lime and the alum and S02. This reaction is completed in a. relatively short time so that the mixture may be used promptly after it. has been formed. The usual seasoning or aging period required in the production of satin white may be eliminated. Moreover, the increased fluidity of the improved product renders it more suitable for use as a coating pigment in machine coating operations. Thus, this product may be used to advantage in the machine coating of ground wood papers and when so used the age deterrent property of calcium sulflte will serve to prolong the life of the paper.

In another typical operation, in accordance with the invention, a solution of alum, consist- This mixture, when utilized in a coating-- ing of 30% alumand water by weight, wasfirst formed and this solution was then saturated with S02. It was found capable of dissolving 50 grams of sulfur dioxide per liter of solution.

-This solution of alum and S0: was then added to milk of lime containing such an amount of lime as to provide a ratio of CaO to S0: of 1.75. Upon analysis of the resulting mixture, it was found produced from the same raw satisfactory. There is to contain 9.7% air-dry basis. This mixture was also found to have excellent properties for coating pigment purposes, superior to those of straight satin white materials. Its viscosity was considerably lower, its brightness higher, and its gloss substantially the same. The casein requirement of the improved mixture was about 27 as compared with 30% for the straight satin white, in providing compositions safe to a No. 5A Dennison graded wax when applied incoatings of the same weight.

As a result of various operations underdifferent conditions, it has been found that a substantial improvement over ordinary satin white -is produced by mixtures containing anywhere between 1 and 10% calcium sulflte with the balance substantially all satin white. It has been desirable, moreovenas a result of these operations, to maintain the CaO to S0: ratio above mentioned'between about 1.6 and 2.00. Preferably, it should be maintained between 1.75 and 1.8. Within this narrower range the greatest improvement in fluidity or viscosity is found. In all instances the viscosity of the mixture was found to be lower than for straight satin white .but when combined with casein, the viscosity of the mixture was in some instances found higher than for straight satin white with casein. If the indicated ratio is carried below 1.6 or above 2.00, the viscosity of the mixture with casein seemsjto be unpredictable and at times it is unsome advantage, also, from the standpoint of gloss and brightness in having the ratio within the narrower limits specifled. The casein requirement of the improved mixture has been found in some instances to be reduced as much as 20%, or even 30%, below that of straight satin white.'

While it is ordinarily preferred to .flrst dissolve the alum in the water, thereby enabling the use of hot water which may then be cooled solution. For example, a strong solution of sulfurous acid or of calcium bisulflte may be separately formed and added to a saturated alum solution.

In the practice of the present invention, the iron in the alum, which would ordinarily discolor the satin white produced in the reaction between the alum and the milk of lime, is apparently converted into some-form in which it produces no discoloration in the improved mixture.

This product is as whiteas if iron-free alum had been used. Possibly this is dueto the reducing action of the SO: which may convert the ferric ions to ferrous ions whosesalts produce little or no discoloration of the final product.

It is preferred to incorporate between 3 and the mixture, but greater 10% of calcium sulflte in or smaller amounts may be employed, if desired. with good results. For example, as little as 1% of the mixture may be calcium sulflte with good results, particularly if the CaO to S0; ratio is relatively high, say around 1.8. .Larger amounts, in excess of 10%. of calcium sulflte are also dvantageous but it is difficult and more expensive .to co-precipitate a greater percentage with the satin white. If a relatively small amount is decalcium sulflte, by weight, on an I 2,310,093 sired, a correspondingly small amount of S: is v greater quantity of water is then fully saturated with 802. The alum solution may, for example, consist of'only 20% alum-in.80%' water. The same result may be accomplished by maintaining a higher percentage of SO: in solution in a saturated or more concentrated alum solution through the use of pressure.

Preferably, the milk of lime employed in the process is at a concentration, around 150 g./l., at

which it may be readily pumped, stirred and screened. Good results may, however, be obtained by the use of milk of lime at higher or lower concentrations (say between 100 and 200 grams per liter) so long as too great a burden is not placed upon the mixing equipment and the conditions are such that a relatively fine precipitate of calcium sulfite is formed.

By maintenance of the proper ratio of CaO to $03, the pH of the resulting mixture may be kept well on the alkaline side. It should have a pH above and preferably in the neighborhood of 11.6 or even 11.8.

While the invention has been described in considerable detail in the foregoing, it will be understood that various changes may be made in the method and in the resulting product without departing from the general principles and scope of the invention. The terms and expressions employed herein have been used as terms of description and not of limitation.

What I claim is:

1. A method of producing a pigment substance which comprises dissolving alumin water, dissolving sulfur dioxide in the resulting alum solution, and-adding the solution of alum and sulfur dioxide to armilk of lime suspension.

sulfur dioxide into said solution until it is substantially saturated with said sulfur dioxide, and

then adding said solution to a milk of lime sus- 5. A method of producing a pigment substance which comprises forming a non-saturated solution of alum, substantially saturating said solution with $02, and then adding .said solution to a milk of lime suspension.

6. A method of producing a pigment substance which comprises forming a substantially saturated solution of alum with sulfur dioxide in solution therein, and adding said solution to a milk of lime suspension.

7. A method of producing a pigment substance which comprises forming a solution of alum containing between and 50% alum by weight, dissolving sulfur dioxide in said solution, and then 2; A method of producing a pigment substance which comprises dissolving alum and sulfur dioxide in water and adding the resulting solution to a, milk of lime suspension.

3. A method of producing a pigment substance which comprises forming a substantially-saturated solution of alum and sulfur dioxide in water and adding the same to a suspnsion of lime to co-precipitate satin white and calcium sulfite.

4. A method of producing a pigment substance which comprises forming a substantially saturated solution of alum, introducing gaseous adding said solution to a milk of lime suspension. 8. A method of producing a pigment substance which comprises forming a solution of alumcontaining between 20 and alum by weight, dissolving sulfur dioxide in said solution, and then adding said solution to a milk of lime suspension,

the ratio of the CaO to the S03 present in'said' suspension upon the addition of said solution thereto'being between 1.6 and 2.00.

9. A method of producing-a pigment substance I which comprises co-precipitating satin white and calcium sulfite by reacting a milk of lime suspension with a solution of alum and sulfur dioxide.

10. A method of producing a pigment substance which comprises co-precipitating satin white and calcium sulfite by reacting a milk of lime suspension with a solution of alum and sulfur dioxide, the ratio of 0210 to S0: in said reaction being between 1.6 and 2.00.

11. As a coating pigment a cit-precipitated intermixture of satin white and calcium sulfite in a finely divided state.

12. As a coating pigment a finely divided, coprecipitated mixture of satin white with iron impurities in a ferrous state and calcium sulfite.

13. As a coating pigment a finely divided, coprecipitated mixtureof satin white with iron impurities in a ferrous state and calcium sulfite, the calciumsulfite comprising between 1 and 10% of the mixture on an air-dry basis. v

.14. As a coating pigment a finely divided, co-

precipitatedmixture of satin white and calcium sulfite, the calcium sulfite being between 1 and 10% of the mixture, and said mixture having a ratio of CaO to S0: of-bet ween 1.6 and 2.00.

. GERALD HAYWOOD. 

